Factors controlling the abundance of rainbow trout in the Colorado River in Grand Canyon in a reach utilized by endangered humpback chub

We estimated the abundance, survival, movement, and recruitment of non-native rainbow trout (Oncorhynchus mykiss) in the Colorado River in Grand Canyon to determine what controls their abundance near the Little Colorado River (LCR) confluence where endangered humpback chub (Gila cypha) rear. Over a 3-year period, we tagged more than 70 000 trout and recovered over 8200 tagged fish. Trout density was highest (10 000-25 000 fish.km(-1)) in the reach closest to Glen Canyon Dam, where the majority of trout recruitment occurs, and was 30- to 50-fold lower (200-800 fish.km(-1)) in reaches near the LCR confluence similar to 100 km downstream. The extent of rainbow trout movement was limited with less than 1% of recaptures making movements greater than 20 km. However, because of high trout densities in upstream source areas, this small dispersal rate was sufficient to explain the threefold increase in the relatively small population near the LCR. Reducing dispersal rates of trout from upstream sources is the most feasible solution to maintain low densities near the LCR to minimize negative effects of competition and predation on humpback chub.